CN114280361A - Weak current measuring device - Google Patents
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- CN114280361A CN114280361A CN202111635869.XA CN202111635869A CN114280361A CN 114280361 A CN114280361 A CN 114280361A CN 202111635869 A CN202111635869 A CN 202111635869A CN 114280361 A CN114280361 A CN 114280361A
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Abstract
The invention relates to a weak current measuring device, which comprises a detection component, a conversion component, a power supply component and a calibration component, wherein the calibration component is used for correcting detection data of each component and is configured to: and selecting manual calibration or automatic calibration based on the current source condition of the current device to be detected. The automatic calibration includes: and adjusting to each detection gear, and respectively carrying out a plurality of times of detection, wherein the detection mean value is used as a correction standard of the detection data of each component. The weak current measuring device is designed based on a high resistance method principle, and the calibration assembly is designed to be used as a front assembly of the device, so that a measuring range changing circuit is integrated between the calculating unit and the current signal conversion circuit while the device is ensured to have higher speed when detecting weak current, a user can adjust the measuring range to a proper range according to the approximate range of the current to be measured, the measuring accuracy is ensured, the device has high performance and practicability at the same time, and a foundation is laid for the practical application of the radioactive drug activity field measurement.
Description
Technical Field
The invention relates to the technical field of infinitesimal current detection, in particular to a device for measuring infinitesimal current.
Background
Generally, a current below the nA level is defined as a weak current, and detection of the weak current is often required to be carried out in scientific research works in various fields such as nuclear technology application, ionizing radiation, electrochemistry and materials science. In particular nuclear medicine, the high rate of disintegration over time of more and more short-lived or ultra-short-lived artificial nuclides used in clinical diagnosis and therapy poses significant difficulties in absolute measurement of activity and in verification of results that need to be performed at any time.
At present, the activity measuring system of a4 pi gamma high-pressure ionization chamber is developed by the Chinese national institute of metrology science (NIM) and is used for the rapid measurement and the quantity value transmission of the activity of nuclide. The measuring system consists of a4 gamma-free high-voltage ionization chamber and a very weak current detection circuit. The extremely weak current detection circuit is a key component of the whole activity measurement system. Aiming at the characteristics of low current background, wide range and the like of the current output of a 4-gamma ionization chamber, a weak current measuring circuit applied to the ionization chamber has the following special technical requirements:
1. the automatic scale calibration function is realized, and the scale calibration operation is simple and convenient;
2. the ionization chamber has fA (1fA ═ 1O-15A) grade background current detection capability;
3. the measurement dynamic range is wide, and dozens of fA to mA orders of magnitude (the range of the weak current signal to be measured in the measurement task of the ionization chamber usually spans multiple orders of magnitude);
4. the detection speed is high, and the field detection is facilitated;
5. the current value detection precision is high and the reading is stable.
The weak current detection is also difficult in that the amplitude of a current signal to be detected is small, so that the current signal is very easily influenced by various interference sources, such as electrical noise, mechanical vibration and an external magnetic field. Moreover, the difference of external temperature, humidity and scene space size also has requirements on the current environment adaptability and volume, so that the current signal conversion method based on large-volume high-value resistance cannot be used.
Aiming at the output current characteristic of the 4 pi gamma ionization chamber, the range of the measurable current value of the extremely weak current detection circuit in the activity measurement system of the 4 pi gamma high-voltage ionization chamber developed by the National Institute of Metrology (NIM) is 0.1 pA-10 hA. The circuit utilizes the principle that a Thomson balance method is used as an electrometer, ensures higher measurement precision and greatly expands the dynamic monitoring range. However, this device also has two disadvantages: firstly, the Thomson equilibrium method ensures the detection precision of extremely weak current by sacrificing the detection response time of small current, and the detection time of measuring the background weak current of an ionization chamber needs several hours, so the current detection speed based on the technology is slow; secondly, a plurality of discrete components with low integration level are used, and the volume of the equipment is equivalent to the size of a household vertical refrigerator. Based on the two points, the device does not have the function of field real-time measurement.
In the related art, the development of the medical level leads to that more and more radionuclides with short half-life and different activities are applied to the clinical diagnosis and treatment of nuclear medicine, higher requirements are put forward on the detection response time and the detection accuracy of an activity meter, and the development of a small-volume radioactive activity meter with higher accuracy, wider activity measurement range and higher response speed is urgent. As a core component of the activity meter, the infinitesimal current detection circuit must have a wider current measurement dynamic range, higher measurement accuracy and faster response speed, so that the infinitesimal current detection circuit which is suitable for field measurement and satisfies the current output characteristic aiming at the 4 pi gamma ionization chamber needs to be designed.
Chinese patent CN101353237B discloses a micro-current detection circuit and a micro-current switching circuit, wherein the micro-current detection circuit comprises a sampling resistor, one end of the sampling resistor is connected with an identification circuit and the detection end of a detected circuit, and the other end of the sampling resistor is connected with a power supply and a clamping circuit; the identification circuit is connected with the clamping circuit; the identification circuit and the clamping circuit both comprise triodes; the identification circuit is the same as the triode model in the clamping circuit; the emitter bias voltage of the triode in the clamping circuit is power supply voltage-clamping voltage, and the emitter bias voltage of the triode in the identification circuit is detection end voltage-clamping voltage; the clamp voltage is the voltage at the junction of the identification circuit and the clamp circuit. Because the identification voltage of the identification circuit is about 0.2V, the micro-current detection circuit can detect the mA-level current flowing in the sampling resistor. In the micro-current switching circuit, the micro-current detection circuit is connected with a switching circuit. And controlling the switching circuit according to the detection current result to realize circuit switching. The defects of the patent are that: the detection precision is too low to reach the mA-level current, the detection cannot be used in the extremely weak current detection of a4 pi gamma high-voltage ionization chamber, and the patent lacks the functions of range switching and calibration.
Chinese patent CN101907654A discloses a weak current detection device for detecting large dynamic weak current, requiring real-time recording and displaying of measurement data, and being capable of being used for radiation detection, which includes a dc power supply module, a weak current amplification and I-V conversion circuit, an a/D conversion circuit, a central control unit for completing data processing, range switching and data communication functions, a range selection switch circuit for realizing automatic range selection by selecting a feedback path of a feedback network, and a user terminal. The invention realizes automatic range switching, adopts a T-shaped resistance network structure in a feedback network of a weak current amplification and I-V conversion circuit U2, improves the equivalent resistance of the feedback network, and overcomes the temperature drift and capacitive reactance influence caused by common high resistance, thereby solving the technical problems of small range, slow response, low resolution and the like of the existing weak signal measuring equipment, and ensuring that the precision reaches below 0.1pA and the resolution is 10 fA. The defects of the patent are that: although the patent has a range switching function and can achieve detection accuracy of below 0.1pA, the range switching circuit is only a superposition of a plurality of single range switching circuits, and a solution is not provided for various problems such as increase of calibration difficulty and introduction of noise and interference which may be generated by the superposed range switching circuits. And do not have automatic calibration, protection, shielding electromagnetic signal interference, filtering internal noise interference, the introduction of noise signal of suppression power and ground and when guaranteeing to eliminate noise interference, can carry out multiple complex functions such as communication transmission, for 4 pi gamma high pressure ionization chamber activity measurement system, too crude, it is not suitable for.
Furthermore, on the one hand, due to the differences in understanding to the person skilled in the art; on the other hand, since the inventor has studied a lot of documents and patents when making the present invention, but the space is not limited to the details and contents listed in the above, however, the present invention is by no means free of the features of the prior art, but the present invention has been provided with all the features of the prior art, and the applicant reserves the right to increase the related prior art in the background.
Disclosure of Invention
Aiming at the defects of the prior art, the technical scheme of the invention is to provide a weak current measuring device, which comprises: detection component, conversion component and power supply component, characterized in that, the device still includes: a calibration component for correction of component detection data, the calibration component configured to: based on whether the current source of the current device to be detected is connected to the device to be detected or whether a calibration signal is provided, the selection of manual calibration or automatic calibration is performed, wherein the automatic calibration comprises: and on the basis of each detection gear of a range change circuit for widening the range in the detection assembly, sequentially detecting each detection gear for a plurality of times in a mode of not inputting current, and taking the detection mean value as a correction standard of detection data of each assembly. The weak current measuring device is designed based on a high-resistance method principle, and the calibration assembly is designed to be used as the front detection assembly of the device, so that the device has high detection speed of weak current, and meanwhile, the measuring accuracy is guaranteed, and the device has high performance and practicability.
According to a preferred embodiment, the calibration module runs before the detection module, and selects whether the instrument is calibrated, and if not, the detection module runs and detects the current value; if the calibration is carried out, selecting manual calibration or automatic calibration based on the current source condition of the current device to be detected, and if the calibration is manual calibration, carrying out accurate calibration based on zero level calibration and full-scale calibration of each gear by using a current source input current value; if the calibration is automatic calibration, on the basis of not carrying out current input, carrying out detection on each detection gear for a plurality of times, and taking the detection mean value as a correction standard of detection data of each component. The calibration assembly is used for carrying out data calibration on the small current measuring device, the problem that the price of a calibration extremely weak current source is high is solved, the problem that each current detection instrument is difficult to ensure to be provided with a current source capable of providing a calibration signal is solved, after the rapid and high-precision calibration is provided, an implementation basis is provided for the subsequent range precision design and noise reduction design of the small current measuring device, and the problem that the calibration difficulty is increased due to the fact that the small current measuring device is smaller in size, less in noise and more accurate in measuring range is solved.
According to a preferred embodiment, the detection assembly comprises a current signal conversion circuit, a filter circuit, an analog-to-digital conversion circuit and a range change circuit, wherein the current signal conversion circuit comprises an electrometer-grade operational amplifier; the input end of the static electricity meter-level operational amplifier is used for receiving a current signal to be measured, the output end of the static electricity meter-level operational amplifier is connected to the filter circuit, a first resistor is arranged at the input end of the current signal to be measured on the basis of a high impedance principle, a second resistor is arranged at the output end of the static electricity meter-level operational amplifier, and the first resistor and the second resistor are used for converting the current signal to be measured into a voltage signal and improving the current detection response speed in a mode of stabilizing the current signal to be measured. The current to be measured is converted into a voltage signal in a proper range through the first resistor with high resistance, and the magnitude of the current to be measured is calculated through the detected characteristics of the voltage signal and the magnitude of the resistance value of the feedback resistor, so that the response speed is very quick, the obtained current value is stable, the time consumed by digital signal processing is reduced through the filter circuit, and the function of quick measurement is realized.
According to a preferred embodiment, the conversion assembly is configured to process a digital signal output by the analog-to-digital conversion circuit and output a detected value of a current signal, and includes a calculation unit, a communication circuit and a monitoring circuit, the calculation unit is connected to the analog-to-digital conversion circuit, the communication circuit and the monitoring circuit are electrically connected to the calculation unit, respectively, and the calculation unit is in communication connection with an upper computer through the communication circuit.
According to a preferred embodiment, the range changing circuit includes a plurality of shift circuits for providing detection of the extremely wide range circuit, the plurality of shift circuits form a resistance network for changing the range, and the plurality of shift circuits control whether to be connected to the feedback network of the current signal conversion circuit based on the level signal of the computing unit. The resistor network for switching the measuring range is formed by the fixed resistors, the measuring range can be switched quickly and accurately, and the aim of quickly measuring the wide measuring range is fulfilled. The calibration component is matched with the range changing circuit, so that the problem of calibration difficulty improvement caused by the fact that a plurality of fixed value resistors are connected, namely, a gear circuit is expanded is solved, and the change of the calibration degree of the fixed value resistors cannot be influenced by the connection of the fixed value resistors on the premise that the calibration degree meets the activity measurement of the 4 pi gamma ionization chamber.
According to a preferred embodiment, the device further comprises a shielding component and a protection component, wherein a PCB of the device is made of epoxy resin, a protection ring in the protection component is arranged at an input end of a current signal to be measured on the PCB, the protection ring is arranged in a manner of surrounding the input end of the current signal to be measured and is used for improving the performance of the detection component, and a protection layer is arranged on a lower layer of the PCB.
According to a preferred embodiment, the shielding assembly is disposed between the high-impedance input and the interference source, and includes a first shielding body and a second shielding body, wherein the first shielding body is connected to a protection level pin of the electrometer-level operational amplifier, the second shielding body is electrically connected to the low-impedance source, the shielding assembly is of an inner-outer layered structure, the first shielding body is located on an inner layer, and the second shielding body is located on an outer layer. Wherein the first shield and the second shield are used for reducing electromagnetic signal interference. The first shield connected to the protection level pin and the second shield connected to the low impedance source solve the problem of the impedance of the electrometer-level op-amp, such as the movement of hands or paper, and at the same time reduce the interference of various electromagnetic signals in the environment.
According to a preferred embodiment, the filter circuit comprises an eight-order active low-pass filter for analog signal building, a programmable filter for ADC and a filtering algorithm for digital signal noise. The filter circuit consists of the eight-order active low-pass filter, the programmable filter and a filter algorithm and is used for reducing internal noise interference which cannot be inhibited by the protection component and the shielding component, such as mechanical vibration noise, thermal noise of components, power frequency interference and other noise, and the precision of current value measurement is greatly improved.
According to a preferred embodiment, the power supply assembly is provided with an analog power supply and a digital power supply based on limiting the introduction of noise signals from the power supply and the ground signal, the analog power supply and the digital power supply separately supply power to the detection assembly and the conversion assembly, respectively, wherein after the digital power supply is subjected to level conversion in a two-way isolated manner, one way supplies power to the calculation unit and the monitoring circuit, and the other way supplies power to the communication circuit. The analog power supply and the digital power supply are used for completely isolating the power supply of the analog signal, the digital signal and the communication signal, so that the problem that the noise generated by a noise source is inhibited at the source of the noise after the protection, the shielding and the filtering is solved, and the introduction of the noise signal from the power supply and the ground signal is limited to a great extent.
According to a preferred embodiment, the analog-to-digital conversion circuit comprises a reference voltage source and an analog-to-digital converter; the reference voltage source is connected to a REF pin of the analog-digital converter, an output end of the analog-digital converter is connected to the conversion assembly, and the analog-digital conversion circuit and the conversion assembly are in communication connection through a signal transmission circuit designed based on a magnetic isolator. The signal transmission circuit is used for communication after power supply isolation of three parts of analog signals, digital signals and communication signals, and ensures that the analog circuit is free from noise interference caused by frequent level conversion of the digital signals and simultaneously ensures that all parts can perform good communication transmission.
The invention has the beneficial technical effects that:
(1) the weak current measuring device is designed based on a high resistance method principle, and the calibration assembly is designed to be used as the front detection assembly of the device, so that the device can detect the weak current at a high speed, and meanwhile, the range change circuit is integrated between the calculation unit and the current signal conversion circuit, so that a user can adjust the range to a proper range according to the approximate range of the current to be measured, and the measuring accuracy is ensured. The design scheme of the invention ensures that the device has high performance and practicability at the same time, and lays a foundation for the practical application of the activity field measurement of the radiopharmaceuticals;
(2) the calibration component is used for carrying out data calibration on the micro current measuring device, so that the problem that the price of a very weak current source is high during calibration, and the problem that each instrument for current detection is difficult to ensure to be provided with a current source capable of providing a calibration signal for the instrument is solved, and after rapid and high-precision calibration is provided, an implementation basis is provided for the subsequent range precision design and noise reduction design of the micro current measuring device, and the problem that the calibration difficulty is increased for obtaining a smaller volume, less noise and a more precise measuring range is solved, and the calibration component is not only an additional function;
(3) the input end of the current signal to be measured is provided with the first resistor, the output end of the electrostatic meter-level operational amplifier is provided with the second resistor, the current signal to be measured is converted into a voltage signal, and the current detection response speed is improved in a mode of stabilizing the current signal to be measured. The measured current is converted into a voltage signal in a proper range through the first resistor with high resistance value, and the magnitude of the measured current is calculated through the detected characteristics of the voltage signal and the magnitude of the resistance value of the feedback resistor, so that the response speed is very quick, the obtained current value is stable, the time consumed by digital signal processing is reduced by the filter circuit, and the function of quick measurement is realized;
(3) through a plurality of gear circuits of the range changing circuit, a resistance network for switching the range is formed by a plurality of fixed value resistors, the detection range can be switched quickly and accurately, and the purpose of quick measurement of the wide range is achieved. The calibration assembly is matched with the range changing circuit, so that the problem of calibration difficulty improvement caused by the fact that a plurality of fixed value resistors are connected, namely, a gear circuit is expanded is solved, and the change of the calibration degree of the fixed value resistors cannot be influenced by the connection of the fixed value resistors on the premise that the calibration degree meets the activity measurement of a4 pi gamma ionization chamber;
(4) the control part of the relay in the range changing circuit is powered by a digital power supply, and the digital signal output by the calculating unit is also isolated from the analog signal part by two-way isolation design of the digital power supply, so that noise interference caused by the access of a plurality of range changing circuits can be ignored;
(5) the performance of the detection assembly is effectively improved in a mode of surrounding a target signal to be protected by the protection ring arranged at the input end of the measured current signal and the protection layer arranged on the lower layer of the PCB;
(6) the shielding assembly arranged between the high-impedance input and the interference source and the inner and outer layered structures of the shielding assembly are adopted, and the first shielding body connected to the protection level pin and the second shielding body connected to the low-impedance source solve the problem that the operation and amplification at the level of the electrometer are influenced by the non-resistance force, such as the movement of hands or paper, and simultaneously reduce the interference of various electromagnetic signals in the environment;
(7) the three filtering modes respectively comprise an eight-order active low-pass filter built for analog signals, a programmable filter inside an ADC (analog-to-digital converter) and a software filtering algorithm for digital signal noise, so that internal noise interference which cannot be inhibited by the protection assembly and the shielding assembly, such as mechanical vibration noise, thermal noise of components, power frequency interference and other noise, is reduced, and the precision of current value measurement is greatly improved;
(8) the power supply assembly is divided into an analog power supply and a digital power supply according to the modes of supplying power by analog signals and supplying power by digital signals, wherein after the digital power supply is subjected to level conversion in a two-way isolation mode, one way supplies power to the computing unit and the monitoring circuit, and the other way supplies power to the communication circuit. The power supply assembly designed in the way solves the problem that after the protection, the shielding and the filtering, the noise generated by the noise source is restrained at the noise source, and the introduction of noise signals from power and ground signals is limited to a great extent;
(9) the invention designs a signal transmission circuit, and the SPI bus digital signal output of the ADC in the analog circuit can still communicate with the computing unit through the magnetic isolator, so that the design is matched, the analog circuit is prevented from being interfered by noise caused by frequent level conversion of digital signals, and simultaneously, the parts of each part can carry out good communication transmission.
Drawings
FIG. 1 is a schematic circuit block diagram of a preferred embodiment of a weak current measuring device of the present invention;
FIG. 2 is a schematic view of a connection frame of a preferred embodiment of a weak current measuring device of the present invention;
FIG. 3 is a logic diagram of the calibration assembly of the present invention;
FIG. 4 is a circuit schematic of the current signal conversion circuit of the present invention;
FIGS. 5A-5D are circuit schematic diagrams of the low pass filter of the present invention;
FIG. 6 is a circuit schematic of the analog to digital conversion circuit of the present invention;
FIG. 7 is a circuit schematic of the range change circuit of the present invention;
FIG. 8 is a circuit schematic of the digital power supply of the present invention;
FIG. 9 is a schematic diagram of the structure of the guard ring on the PCB of the present invention;
fig. 10 is a circuit schematic diagram of a signal transmission circuit of the present invention.
List of reference numerals
1: calibrating the component; 2: a detection component; 3: a conversion component; 4: a power supply component; 5: a shielding assembly; 6: a signal source; 201: a current signal conversion circuit; 202: a filter circuit; 203: an analog-to-digital conversion circuit; 204: a range changing circuit; 301: a calculation unit; 302: a communication circuit; 303: a monitoring circuit; 304: and (4) an upper computer.
Detailed Description
The following detailed description is made with reference to the accompanying drawings.
Examples
The application relates to a weak current measuring device, including: detection component, conversion component and power supply component, characterized in that, the device still includes: a calibration component for correction of detection data of each component, the detection component comprising a current signal conversion circuit, a filtering circuit, an analog-to-digital conversion circuit, and a range change circuit, the calibration component configured to: based on the current source condition of the current device to be detected, selecting manual calibration or automatic calibration, wherein the automatic calibration comprises: and adjusting to each detection gear, and respectively carrying out a plurality of times of detection, wherein the detection mean value is used as a correction standard of the detection data of each component. Fig. 1 shows a circuit block diagram of an exemplary embodiment of a weak current measuring device. The weak current measuring device is designed based on a high resistance method principle, and the calibration assembly is designed to be used as the front detection assembly of the device, so that when the device detects weak current, the device has higher speed, and meanwhile, the range change circuit is integrated between the calculation unit and the current signal conversion circuit, so that a user can adjust the range to a proper range according to the approximate range of the current to be measured, and the measurement accuracy is ensured. The design scheme of the invention ensures that the device has high performance and practicability at the same time, and lays a foundation for the practical application of the field measurement of the activity of the radiopharmaceutical.
According to a preferred embodiment, the calibration module runs before the detection module, and selects whether the instrument is calibrated, and if not, the detection module runs and detects the current value; if the calibration is carried out, selecting manual calibration or automatic calibration based on the current source condition of the current device to be detected, and if the calibration is manual calibration, inputting a current value by using a current source meter to carry out accurate calibration based on zero level calibration and full-scale calibration of each gear; if the calibration is automatic calibration, on the basis of not carrying out current input, carrying out detection on each detection gear for a plurality of times, and taking the detection mean value as a correction standard of detection data of each component. Specifically, the calibration component operates before the detection component, initialization parameter configuration processing is performed on each component during operation, a user sends out an instruction whether to perform calibration through the calculation unit, and if not, the detection component operates to directly detect the current value. And if the calibration is carried out, selecting whether to carry out automatic calibration or not based on the current source condition of the current device to be detected, if not, running manual calibration, and inputting a proper current value to carry out accurate calibration based on zero level calibration and full-scale calibration of each gear by using a current source meter. If automatic calibration is carried out, automatic calibration is carried out under the condition that current input is not carried out (namely the input current value is zero), the calibration component is sequentially adjusted to each detection gear and is subjected to detection for a plurality of times, then the detection mean value is written into the offset register, the automatic calibration is completed, the detection component can be started, and in the detection process of the detection component, the data of the offset register can correct the detection data, so that the purpose of automatic calibration is realized. The offset register is an eight byte offset register.
It should be noted that there are many reasons for the calibration, and in the embodiment of the present invention, the infirm current detection instrument generally needs to be calibrated periodically to ensure the accuracy of the measured data, mainly due to the uncertain dc bias of the internal devices of the instrument, and due to the difference of the usage environment and the aging factors of the internal devices of the instrument. However, the infinitesimal current source for calibrating the current detection device is expensive, so that it is difficult to ensure that each current detection device is provided with a current source for providing a calibration signal, and the usability of the current detection device is greatly improved if the device has an automatic calibration function. According to the scheme, after the digital current source meter is used for manually calibrating each measuring range of the current detection device, an automatic calibration function is added, and a corresponding control logic is designed to realize the selection of the calibration function. The auto-calibration function may cause the current sensing device to zero each range without a current source meter. The digital current source meter is, for example, Model-6430 manufactured by Keithley, but is not limited thereto.
The specific operation steps of the calibration assembly are as follows:
s1: the calibration component operates before the detection component, and performs initialization parameter configuration processing on each component during operation;
s2: the calculating unit sends out a command of whether to carry out calibration, if not, the step S6 is executed, and if so, the step S3 is executed;
s3: selecting whether to perform automatic calibration or not based on the current source condition of the current device to be detected, if not, executing S4, and if so, executing S5;
s4: running manual calibration, and inputting a proper current value by using a current source meter to carry out accurate calibration based on zero level calibration and full-scale calibration of each gear;
s5: when the current is not input (namely the input current value is zero), automatic calibration is carried out, the calibration component is sequentially adjusted to each detection gear and is used for carrying out detection for a plurality of times, and then the detection mean value is written into the bias register;
s6: and the detection component operates to detect the current value and correct the detection data according to the offset register to obtain a final detection value.
According to a preferred embodiment, the current signal conversion circuit comprises an electrometer-level op-amp; the input end of the static electricity meter-level operational amplifier is used for receiving a current signal to be measured, the output end of the static electricity meter-level operational amplifier is connected to the filter circuit, a first resistor is arranged at the input end of the current signal to be measured on the basis of a high impedance principle, a second resistor is arranged at the output end of the static electricity meter-level operational amplifier, and the first resistor and the second resistor are used for converting the current signal to be measured into a voltage signal and improving the current detection response speed in a mode of stabilizing the current signal to be measured. Specifically, the first resistor is disposed at the RES + position of the circuit, and the second resistor is disposed at the RES-position of the circuit, which together constitute the current signal conversion circuit. The current to be measured is input through the terminal J1 and converted into a voltage signal in a suitable range through the first resistor with a high resistance value, namely, the voltage to ground of the TP1 detection point. And calculating the magnitude of the current to be measured by detecting the characteristics of the voltage signal and the magnitude of the resistance value of the feedback resistor. The high resistance method has an advantage in that the response speed is very fast, and as long as a stable current value flows in the resistor, a stable voltage value is detected across the resistor. Therefore, the response time of the current detection device using the high-impedance method as the basic principle mainly depends on the speed of other components (such as an operational amplifier) in the circuit reaching the new balance and the time consumed by the post-signal processing. After the current signal conversion circuit is constructed based on the high-resistance method, the filter circuit is added to reduce the time consumed by digital signal processing. In practical test, the measured current with the size of 100fA does not exceed 15 seconds from the beginning to detect the stability of the readings, and the function of quick measurement is realized.
According to a preferred embodiment, the conversion assembly is configured to process a digital signal output by the analog-to-digital conversion circuit and output a detected value of a current signal, and includes a calculation unit, a communication circuit and a monitoring circuit, the calculation unit is connected to the analog-to-digital conversion circuit, the communication circuit and the monitoring circuit are electrically connected to the calculation unit, respectively, and the calculation unit is in communication connection with an upper computer through the communication circuit. Preferably, the calculation unit can be an MCU minimum system, and the communication circuit realizes the reliable transmission of ionization chamber current data at a longer distance through an RS485 bus, thereby being beneficial to realizing remote data monitoring.
According to a preferred embodiment, the range changing circuit includes a plurality of shift circuits for providing detection of the extremely wide range circuit, the plurality of shift circuits form a resistance network for changing the range, and the plurality of shift circuits control whether to be connected to the feedback network of the current signal conversion circuit based on the level signal of the computing unit. The invention switches the detection range rapidly and accurately by the designed range change circuit, thereby achieving the purpose of rapid measurement in wide range. The accurate current measurement range of the invention is 0.05pA-0.5mA, which reaches 10 orders of magnitudeVery wide range current measurement. The principle is as follows: the current measurement span range of a single fixed value resistance is 103(i.e., the ratio of the highest measured current value to the lowest measured current value is 103) In the circuit with extremely high precision requirement, the current measurement span range of a single constant value resistor is reduced to 102And even smaller. It does not exist in practice if only one fixed resistor is relied upon to perform a wide range of current measurements. Because the smaller the measuring current range of a single constant value resistor is, the more the accuracy can be ensured, and the wider the measuring range spliced together can be ensured by the feedback resistor network for completing the current measurement comprising the resistors with more resistance values. The invention has the advantages that eight fixed value resistors are connected to form a resistor network for switching the measuring range, the average measuring span range of each fixed value resistor is about 20, and the measuring precision is effectively ensured.
It should be noted that the more the number of constant value resistors that are not simply connected, the higher the measurement accuracy. The more the constant value resistors are connected, in order to ensure the detection accuracy of the device, the calibration difficulty of the device is increased, and the invention connects a plurality of constant value resistors through the arrangement of the calibration assembly on the premise that the calibration degree meets the activity measurement of the 4 pi gamma ionization chamber, so as to meet the requirement of the detection accuracy. Namely, the influence of calibration difficulty caused by the fact that the gear circuit is expanded is solved through the arrangement of the calibration component.
Specifically, the specific circuit components of each range changing circuit include a feedback resistor, a relay and a switch tube. Fig. 7 shows a specific structure of the range changing circuit, and whether the feedback resistor R5 is connected to the feedback network of the current signal conversion circuit is controlled by the level signal resrelax _1 output by the computing unit by using the clarinet relay RLY1 which has a mechanical structure, a small volume, a high switching speed and a long service life. The control part of the relay is powered by a digital power supply, the digital power supply is a power supply with two isolated paths, and a digital signal output by the calculating unit is also isolated from the analog signal part, so that noise interference introduced by the circuit design can be ignored, and the influence on the measurement precision is avoided to the greatest extent while the range switching work is completed. The invention eliminates the noise problem caused by accessing a plurality of range changing circuits by a plurality of noise reduction means of separately supplying power by a power supply, carrying out two-way isolation power supply by a digital power supply and isolating a digital signal and an analog signal of a calculating unit.
According to a preferred embodiment, the device further comprises a shielding component and a protection component, wherein the PCB board of the device is made of epoxy resin material, because: because this device is used for detecting the extremely weak measured current signal, even the material that impedance is the highest among the prior art can't realize insulating in the real sense in the practical application yet, consequently the PCB material in the current detection circuit can become the electric leakage medium that can not neglect, and the insulating ability of PCB material directly concerns the conversion precision of current signal converting circuit. Currently, the commonly used PCB material with better insulation property is epoxy resin, teflon, sapphire and polystyrene. Wherein, the sapphire is not adopted because of high cost and low economic value; although polystyrene belongs to an excellent insulating material, the solvent resistance, oxidation resistance and heat resistance of polystyrene are poor, so that the service life of polystyrene is influenced in the using process; the polytetrafluoroethylene has high insulation resistance and strong corrosion resistance and heat resistance, but the polytetrafluoroethylene material is soft and difficult to process, and the polytetrafluoroethylene material is easy to spontaneously form current interference under mechanical stress to influence the measurement stability, so that the polytetrafluoroethylene material is not suitable for being used as a PCB material of extremely weak current detection equipment. In conclusion, the PCB is finally customized by adopting the epoxy resin material which has relatively low impedance, stronger corrosion resistance and lower board manufacturing difficulty.
The protection work is the basis for the high impedance work. The target signal to be protected should be surrounded by another conductor driven to the protection voltage. The invention can effectively improve the performance of the measuring circuit by the arranged protective measures, for example, a protective ring in a protective component is arranged at the input end of a current signal to be measured on a PCB, and a protective layer is arranged at the lower layer of the PCB. Specifically, a guard ring is placed at 15 mils (1 mil-0.0254 mm) around the input signal and a protective copper layer is left under the signal.
According to a preferred embodiment, the shielding assembly is disposed between the high-impedance input and the interference source, and includes a first shielding body and a second shielding body, wherein the first shielding body is connected to a protection level pin of the electrometer-level operational amplifier, the second shielding body is electrically connected to the low-impedance source, the shielding assembly is of an inner-outer layered structure, the first shielding body is located on an inner layer, and the second shielding body is located on an outer layer. Since the electrometer-level op-amp is extremely sensitive, hand or paper movement can be easily detected. Such effects are not periodic or predictable, and they may manifest as odd dc drift. In order to reduce the interference of various electromagnetic signals in the environment, the invention introduces the shielding component to shield the current signal conversion circuit. A shielding assembly refers to the addition of a piece of conductive material between the high impedance input and the source of interference, and the conductive material must be electrically connected to a low impedance source, such as signal ground. That is, the second shield is electrically connected to a low impedance source, the physical configuration of the first shield, the second shield and the protective body being nearly identical. Because of this similarity, many protective structures can also provide shielding. In order to ensure the shielding effect, the invention designs an internal and external double-shielding structure in which a first shielding body and a second shielding body are superposed, wherein the first shielding body is connected to a protection level pin of an electrometer-grade operational amplifier ADA4530-1, and the second shielding body is connected with a signal ground.
According to a preferred embodiment, the filter circuit comprises an eight-order active low-pass filter for analog signal building, a programmable filter for ADC and a filtering algorithm for digital signal noise. The protection assembly and the shielding assembly can greatly inhibit external noise interference and improve the measurement performance of the circuit, but the circuit inevitably generates noise such as mechanical vibration noise, thermal noise of components, power frequency interference and the like during operation, so that the influence caused by the noise cannot be completely eliminated. The noise that the circuit itself has or accompanies the input signal is actually present and unavoidable, and cannot be suppressed by protection or shielding, and filtering is an effective means for reducing the noise interference. In order to reduce the noise level of the circuit, the invention designs three filtering means, namely an eight-order active low-pass filter built for analog signals, a programmable filter inside an ADC (analog-to-digital converter) and a software filtering algorithm for digital signal noise. The precision of current value measurement after filtering is extremely high, and the absolute error of the minimum current measurement gear is less than 5fA in practice, so that the measurement requirement is greatly met.
According to a preferred embodiment, the power supply assembly is provided with an analog power supply and a digital power supply based on limiting the introduction of noise signals from the power supply and the ground signal, the analog power supply and the digital power supply separately supply power to the detection assembly and the conversion assembly, respectively, wherein after the digital power supply is subjected to level conversion in a two-way isolated manner, one way supplies power to the calculation unit and the monitoring circuit, and the other way supplies power to the communication circuit. After the protection, masking and filtering, a most important part of the noise suppression operation remains, namely, the suppression processing is performed at the source of the noise. In circuits with extremely high accuracy requirements, power supply signal noise is a non-negligible source of noise signal interference. The ripple magnitude of the signal in the power supply directly affects the noise level of the circuit of the components using the power supply. Power isolation techniques can greatly limit the introduction of noise signals from the power and ground signals. According to the invention, a lithium battery with lower power supply ripple is selected as a power supply, a digital power supply circuit is designed by using an isolation power supply module IB0505S-W75R3 in the lithium battery, two isolated power supply voltages are generated, and the two isolated power supply voltages are subjected to level conversion and then respectively supply power to digital signal parts such as a computing unit and an isolated RS485 bus, so that the complete isolation of three parts of power supplies of an analog signal, a digital signal and an RS485 bus for transmitting data to a display end is realized.
According to a preferred embodiment, the analog-to-digital conversion circuit comprises a reference voltage source and an analog-to-digital converter; the reference voltage source is connected to a REF pin of the analog-digital converter, an output end of the analog-digital converter is connected to the conversion assembly, and the analog-digital conversion circuit and the conversion assembly are in communication connection through a signal transmission circuit designed based on a magnetic isolator. Although the design of completely isolating the power supply of the analog signal, the digital signal and the data transmission part is carried out, a signal transmission circuit is arranged for carrying out communication in order to ensure that the SPI bus digital signal output of an ADC in the analog circuit still needs to be well communicated with a single chip microcomputer. The invention designs the signal isolation transmission circuit to carry out the isolation communication between the single chip microcomputer and the ADC by utilizing the magnetic isolator AD mu M1401BRW, and the design ensures that the analog circuit is free from noise interference caused by frequent level conversion of digital signals and simultaneously ensures that each subassembly can carry out good communication transmission. Finally, in the case of a circuit operating frequency generally low (less than 1MHz), in order to reduce the common impedance of the ground lines, the analog circuit part uses a parallel single-point ground design.
The invention achieves the index that the measurement precision is higher than 0.2% -1% through a series of measures such as reasonably planning a measurement scheme, elaborately designing a shielding system, and pertinently designing a proper filtering noise reduction scheme.
For ease of understanding, the working principle and method of use of a weak current measuring device of the present invention will be discussed.
In order to reduce noise interference and direct current drift brought by the detection circuit, the invention designs the analog circuit part by using low-noise and low-drift devices. The high-value resistor in the current signal conversion circuit is a glass sealed resistor or a metal oxide resistor, and the two resistors have extremely high stability. Most of the resistors selected by the invention have the temperature drift of less than 50 PPM/DEG C, the maximum temperature drift value in the resistors does not exceed 100 PPM/DEG C, and the resistance value is little influenced by the temperature. A mica capacitor with the frequency of about 3pF is connected in parallel to the sampling resistor with the frequency of less than 1G omega to avoid self-oscillation of the circuit, and the mica capacitor has the advantages of small self inductance and leakage loss, high reliability, stable performance, high capacity precision and the like compared with capacitors made of other materials. In the same way, in the filter circuit before entering the ADC, the invention selects the film resistor with the temperature drift as low as 2 PPM/DEG C as the voltage dividing resistor to ensure the stable performance of the circuit. And an ADC (analog-to-digital converter) sampling circuit designed by combining a 24-bit high-precision analog-to-digital converter AD7172-2 with a high-precision voltage reference ADR4525 is selected.
The scheme draws the advantages of various common activity meter designs through extensive investigation, develops a set of small-size current detection equipment with the measurement range of 0.05pA-0.5mA, the absolute measurement error within 0.5pA is less than 5fA, the measurement precision within the other effective measurement ranges is O.2-1%, the response speed of the current value above 0.5pA is less than 2s, and the response speed of the current value below 0.5pA is less than 16s by adopting a proper method and a series of measures, obviously improves various indexes of current measurement, simultaneously has the functions of automatic calibration, automatic background deduction and the like, enables the instrument to have high performance and practicability, and lays a foundation for the practical application of radioactive drug activity field measurement. It should be noted that the automatic calibration function of the present invention provides a fast and high-precision calibration, and also provides a basis for implementing the micro current measuring device, and solves the problem of increasing calibration difficulty for obtaining a smaller volume and a more precise measuring range, rather than just an additional function.
It should be noted that the above-mentioned embodiments are exemplary, and that those skilled in the art, having benefit of the present disclosure, may devise various arrangements that are within the scope of the present disclosure and that fall within the scope of the invention. It should be understood by those skilled in the art that the present specification and figures are illustrative only and are not limiting upon the claims. The scope of the invention is defined by the claims and their equivalents.
Claims (10)
1. A weak current measuring device comprising: detection component, conversion component and power supply component, characterized in that, the device still includes:
a calibration component for correction of component detection data, the calibration component configured to:
based on whether the current source of the current device to be detected is connected to the device to be detected or whether a calibration signal is provided, the selection of manual calibration or automatic calibration is performed, wherein the automatic calibration comprises: and on the basis of each detection gear of a range change circuit for widening the range in the detection assembly, sequentially detecting each detection gear for a plurality of times in a mode of not inputting current, and taking the detection mean value as a correction standard of detection data of each assembly.
2. The weak current measuring device of claim 1, wherein the calibration component operates before the detection component, and selects whether the instrument is calibrated based on whether the current source of the device to be detected is currently connected to the device to be detected or whether a calibration signal is provided,
if the calibration is not carried out, the detection assembly operates and detects the current value;
if the calibration is carried out, the selection of manual calibration or automatic calibration is carried out based on the current source condition of the current device to be detected,
if the calibration is manual calibration, based on zero level calibration and full-scale calibration of each gear, inputting a current value by using a current source meter to perform accurate calibration;
if the calibration is automatic calibration, on the basis of not carrying out current input, carrying out detection on each detection gear for a plurality of times, and taking the detection mean value as a correction standard of detection data of each component.
3. The weak current measuring device of claim 2, wherein said detecting component comprises a current signal converting circuit, a filter circuit, an analog-to-digital converting circuit and a range changing circuit, said current signal converting circuit comprising an electrometer-level op-amp; the input end of the electrometer stage operational amplifier is used for receiving a current signal to be measured, and the output end of the electrometer stage operational amplifier is connected to the filter circuit, wherein,
the current signal conversion circuit is provided with a first resistor at the input end of a current signal to be measured based on a high impedance principle, and is provided with a second resistor at the output end of an electrometer-level operational amplifier, wherein the first resistor and the second resistor are used for converting the current signal to be measured into a voltage signal and improving the current detection response speed in a mode of stabilizing the current signal to be measured.
4. The weak current measuring device of claim 3, wherein the converting component is configured to process the digital signal outputted from the analog-to-digital converting circuit and output a detected value of the current signal, the converting component includes a calculating unit, a communication circuit and a monitoring circuit,
the calculating unit is connected with the analog-to-digital conversion circuit, the communication circuit and the monitoring circuit are respectively electrically connected with the calculating unit, and the calculating unit is in communication connection with an upper computer through the communication circuit.
5. The weak current measuring device of claim 4, wherein said range altering circuit includes a plurality of range circuits for providing ultra wide range circuit sensing,
the plurality of gear circuits form a resistance network for changing the measuring range, and whether the plurality of gear circuits are connected into a feedback network of the current signal conversion circuit or not is controlled based on the level signal of the computing unit.
6. The weak current measuring apparatus according to claim 5, further comprising a shielding member and a protection member, wherein the input terminal of the current signal to be measured is provided with a protection ring in the protection member on the PCB, the protection ring is disposed in a manner of surrounding the input terminal of the current signal to be measured and is used for improving the performance of the detection member, and the lower layer of the PCB is provided with a protection layer.
7. The weak current measuring device as claimed in claim 6, wherein said shielding assembly is disposed between the high impedance input and the interference source, said shielding assembly comprises a first shielding body and a second shielding body, said first shielding body is connected to the protection level pin of the electrostatic discharge level amplifier, said second shielding body is electrically connected to the low impedance source, said shielding assembly is an internal and external layered structure, said first shielding body is located on the inner layer, said second shielding body is located on the outer layer, wherein said first shielding body and said second shielding body are used for reducing the electromagnetic signal interference.
8. The weak current measuring device according to claim 7, wherein the filter circuit comprises an eighth-order active low-pass filter for analog signal building, a programmable filter for ADC and a filter algorithm for digital signal noise, and the filter circuit is composed of the eighth-order active low-pass filter, the programmable filter and the filter algorithm and is used for reducing internal noise interference which cannot be suppressed by the protection component and the shielding component.
9. The weak current measuring apparatus according to claim 8, wherein said power supply means is provided with an analog power supply and a digital power supply based on limiting introduction of noise signals from power and ground signals, said analog power supply and digital power supply supplying power to said detection means and conversion means, respectively, in a manner of separate power supply, wherein,
after the digital power supply is subjected to level conversion in a two-way isolation mode, one way supplies power to the computing unit and the monitoring circuit, the other way supplies power to the communication circuit, and the analog power supply and the digital power supply are used for completely isolating the power supply of three parts, namely an analog signal, a digital signal and a communication signal.
10. The weak current measuring device of claim 9, wherein the analog-to-digital conversion circuit includes a reference voltage source and an analog-to-digital converter; the reference voltage source is connected to the REF pin of the analog-to-digital converter, the output end of the analog-to-digital converter is connected to the conversion component,
the analog-to-digital conversion circuit is in communication connection with the conversion assembly through a signal transmission circuit designed based on a magnetic isolator, and the signal transmission circuit is used for communication after power supply isolation of three parts, namely an analog signal, a digital signal and a communication signal.
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| CN115575688A (en) * | 2022-09-29 | 2023-01-06 | 北京航天计量测试技术研究所 | Weak current detection circuit |
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